What is the reason for using more upper layer burners at low loads in industrial boiler systems?

What is the reason for using more upper layer burners at low loads in industrial boiler systems?
Most superheaters in medium and high pressure boilers mainly rely on convective heat transfer. We all know that the steam temperature characteristic of a convection superheater is that as the load decreases, the steam temperature decreases. When the boiler load is low, it is possible to have the desuperheating water control valve fully closed and the steam temperature still below the lower limit. Although methods such as increasing the excess air coefficient at the furnace outlet or increasing the negative pressure of the furnace can be used to increase the steam temperature, these methods increase the excess air coefficient of the exhaust gas due to the increase in exhaust gas temperature, resulting in an increase in exhaust gas heat loss and a decrease in boiler thermal efficiency.
If the lower burner is stopped as much as possible and the upper burner is used more, the heat absorption of the furnace will decrease due to the upward movement of the furnace flame center, and the smoke temperature at the furnace outlet will increase. Due to the increase in radiation heat absorption and heat transfer temperature difference, the total heat absorption of the superheater increases, resulting in an increase in steam temperature. This method of regulating steam temperature is cost-effective and should be the first method to be used when the steam temperature is low due to low load.